Abstract: Bioloff impulse of LY12 Al irradiated by soft Xray was studied by numerical simulation. Two methods were provided to calculate the bioloff impulse, and the rule of bioloff impulse as a function of original energy fluence was obtained. The calculation results indicate that as the energy fluence increases, the bioloff impulse of LY12 Al increases linearly irradiated by 1 keV soft Xray. The calculation results are agreement with that of some empirical formula and experimental data. Through analyzing the principle of two methods and monitoring each calculation parameter, it is shown that the boiloff impulse irradiated by soft Xray can be calculated using the two methods, but the result gained by ∑pg(t)Δt method is more exact, and accords with practice better.